Use-case
A customer has released an SDK based on Qt 6.5 LTS. It is already being consumed by numerous external applications.

Problem
Users who wish to upgrade to Qt 6.6/6.7 can no longer link to the published SDK, because of a compatibility break in Qt (Qt's compatibility promise is broken).

Steps to reproduce

1. Load the attached example on Linux
2. Build "MyLib" using Qt 6.5 (I tested using Qt 6.5.3 and Qt 6.5.5)
3. Build "MyApp" using Qt 6.6.2 or Qt 6.7.0-RC1

 
Outcomes
MyApp fails to link to MyLib:

/usr/bin/ld: /home/USER/AppAndLib/MyApp/../lib/libMyLib.so: undefined reference to `QFutureInterfaceBase::setContinuation(std::function<void (QFutureInterfaceBase const&)>, QFutureInterfaceBasePrivate*)@Qt_6_PRIVATE_API'
collect2: error: ld returned 1 exit status

Analysis
Qt 6.5 is the broken version - it is incompatible with both Qt<=6.4 and Qt>=6.6. Unfortunately, Qt 6.5 is also the current LTS version. So, Qt 6.5.6 needs to be made compatible with the other currently-supported versions of Qt.